Nutrients and Solubility

Solubility Product Experiment

Key Concepts

Molecular Basis for Water Solubility and Fat Solubility (e.g., of Vitamins)

Polarity of Solvent and Solute

Thermodynamics of Dissolution

Structures and Functions of Key Vitamins

Effect of Olestra (Artificial Fat) on Vitamin Solubility

Quantitative Measures of Mineral Solubility

Solubility Product (Ksp)

Solubility (S)

Calcium in the Body

Role of Calcium in the Body

Solubility and Absorption of Calcium

Control of Calcium Levels in the Body

Vitamins and Minerals as Essential Dietary Components

The bulk of the food that we consume provides us with water, which
accounts for 50% to 70% of our body weight, and the energy-yielding nutrients, such as
carbohydrates (sugars and starches), lipids (fats), and proteins (Figure 1). In addition
to these major nutrients, our bodies require a variety of other molecules and ions to
maintain proper function. These nutrients, which are required in much smaller amounts, are
known collectively as vitamins and minerals.

Figure 1

Carbohydrates, proteins, fats, and water account for most of our nutritional
requirements. Vitamins and minerals are required in much smaller amounts, yet their
contributions to the body's functioning are essential.

Fourteen vitamins have been shown to be essential for normal growth and
health in humans. Vitamins are organic molecules (i.e., molecules
containing the elements C, H, N, or O) that are needed in trace amounts to help catalyze
many of the biochemical reactions in the body. The term "vitamin" derives from
the words "vital amine," because the first vitamins to be discovered contained
an amino group (-NR2, where R is a hydrogen or some carbon-containing
functional group) in their molecular structure. The fourteen vitamins that we know today
do not have any particular structure in common, nor do they share a common function, but
they can be divided into fat-soluble (nonpolar) and water-soluble (polar) molecules. In
general, vitamins do not themselves provide chemical energy or act as biochemical building
blocks for the body. Many vitamins (e.g., the B vitamins) assist enzymes (act as
coenzymes) in activities ranging from vision to growth ability. (Enzymes are proteins or
other molecules that catalyze reactions, i.e., make them go faster, without themselves
being permanently transformed. You will learn about more catalysts and enzymes in the
"Kinetics" experiment and the related tutorial, "Drug Strategies
to Target HIV: Enzyme Kinetics and Enzyme Inhibitors".) Other vitamins, such as
the antioxidants (e.g., vitamin C, vitamin E), help to maintain structures within cells.

Plants and bacteria have the enzymes necessary to synthesize their own
vitamins; however, animals do not have the ability to synthesize vitamins and must consume
them in the diet. (One exception is Vitamin D, which we can synthesize from cholesterol if
we get enough sunlight.) Hence, we obtain our vitamins by eating plants or meat (and diary
products) from animals that have eaten plants.

Minerals are typically defined by nutritionists as
inorganic (not C, H, N, or O) elements, which are used in the body to help promote certain
reactions, or form structures in the body. This definition differs slightly from (is a
subset of) the usual chemical definition of a mineral, which is a naturally-occurring,
nonmolecular solid. (A nonmolecular solid has a lattice structure rather than discrete
molecular units.) We will use the nutritional definition in this tutorial. Minerals are
typically consumed in the form of a salt containing the mineral element and another ion.
For example, the calcium in Tums is in the form of calcium carbonate (CaCO3).
Minerals, like vitamins, perform a wide variety of functions in the body. Some, such as Mg2+
and Zn2+, enable enzymes to function, catalyzing biochemical reactions. Others,
such as Na+, K+, Ca2+, and Cl-, help to
maintain electrical and water balance in the body, transmit nerve impulses, and stimulate
muscle contraction. Still others, such as Ca and P, form the compound hydroxyapatite that
is responsible for bone growth and structure.

Questions on Vitamins and Minerals as Essential Dietary Components

During a severe and prolonged drought, a village is at risk of famine and vitamin
deficiency because many plants, including grass and the village's normal plant source of
vitamin C, have been killed. One relief worker suggests that the villagers could avoid
vitamin C deficiency by killing their cattle and eat the meat, thus obtaining vitamin C
from the meat, instead of from the plants they normally eat for vitamin C. Would this
strategy work to prevent vitamin C deficiency? Briefly, explain your reasoning.

Although hydrogen (H) is one of the most prevalent elements in the body, it is not
considered a "mineral" in the diet. Briefly, explain why hydrogen is not
considered a mineral.

Nutrients Must Be Soluble

In order to use the nutrients that we take in when we eat, we must first
break the food down into its nutritive components. These components are then either
absorbed by the body, or they pass through the intestinal tract and are removed from the
body in the feces. The nutrients that are absorbed pass through the lining of the
intestinal tract into the blood. The blood carries these nutrients to the sites where they
will be reassembled and used by the body. If the nutrients are not used immediately, they
will either be stored for later use, or excreted in the urine. Each of these fates of the
absorbed nutrients (immediate use, storage, or excretion via urine) requires that the
nutrients be soluble. To be transported from the stomach to other parts of the body, the
nutrients must either be soluble in water (the main component of blood plasma), or be
solubilized by some other particles (e.g., proteins) that are carried in the blood.
Nutrients that are stored in the body are typically stored in fat cells, so they must be
soluble in fat. And of course, to be excreted via the urine, a nutrient must be
water-soluble. Hence, understanding the solubility of nutrients in the different
substances of the body is very important for understanding how they can be used or
processed in the body.

Scientists have developed several ways to discuss the important concepts
of solubility. For salts (ionic solids) that dissociate into ions in water, such as the
compounds containing the dietary minerals, a solubility product (Ksp) is
typically given. The solubility product is the equilibrium constant for the dissociation
reaction of the compound into ions in aqueous solution. This quantity is useful, for
instance, in determining which compound containing a given mineral is more soluble, and
hence would be better absorbed as a dietary supplement (e.g., calcium carbonate vs.
calcium sulfate). The solubility of organic molecules, such as the vitamins, is quantified
using a different scale known as Hidebrand solubility parameters (which will not be
discussed in this tutorial). Organic molecules may be soluble in water or in lipids,
depending on the functional groups on the molecule. A vitamin's solubility in water or in
lipids determines where it can be used, and whether it will be stored in fat cells or
excreted from the body if it is not needed for immediate use.

Vitamin Solubility

Molecular Basis for Water Solubility and Fat Solubility

The solubility of organic molecules is often summarized by the phrase,
"like dissolves like." This means that molecules with many
polar groups are more soluble in polar solvents, and molecules with few or no polar groups
(i.e., nonpolar molecules) are more soluble in nonpolar solvents. (You encountered these
concepts in the "Membranes and Proteins" experiment and the related tutorial,
"Maintaining
the Body's Chemistry: Dialysis in the Kidneys".) Hence, vitamins are either
water-soluble or fat-soluble (soluble in lipids and nonpolar compounds), depending on
their molecular structures. Water-soluble vitamins have many polar groups and are hence
soluble in polar solvents such as water. Fat-soluble vitamins are predominantly nonpolar
and hence are soluble in nonpolar solvents such as the fatty (nonpolar) tissue of the
body.

What makes polar vitamins soluble in polar solvents and nonpolar vitamins
soluble in nonpolar solvents? The answer to this question lies in the types of
interactions that occur between the molecules in a solution. Solubility is a complex
phenomenon that depends on the change in free energy (DG) of
the process. For a process (in this case, a vitamin dissolving in a solvent) to be
spontaneous, the change in free energy must be negative (i.e., DG<0).
The green box below describes the thermodynamic processes that govern solubility.

Thermodynamics of Dissolution (Solubilization)

The dissolution of a substance (solute) can be separated into three steps:

The solute particles must separate from one another.

The solvent particles must separate enough to make space for the solute
molecules to come between them.

The solute and solvent particles must interact to form the solution.

The free energy (G) describes both the energetics (i.e., the enthalpyH) and the randomness or probability (i.e., the entropy S) of a process ( DG=DH-TDS,
where T is the absolute temperature). The enthalpy and entropy changes that occur in the
dissolution process are shown in Figure 2, below. In the dissolution process, steps 1 and
2 (listed above) require energy because interactions between the particles (solute or
solvent) are being broken. Step 3 usually releases energy because
solute-solvent interactions are being formed. Therefore, the change in enthalpy
(DH) for the dissolution process (steps 1 through 3) can be
either positive or negative, depending on the amount of energy released in step 3 relative
to the amount of energy required in steps 1 and 2. In terms of the change in
entropy (DS) of the dissolution process, most
dissolution processes lead to a greater randomness (and therefore an increase in entropy).
In fact, for a large number of dissolution reactions, the entropic effect (the change in
randomness) is more important than the enthalpic effect (the change in energy) in
determining the spontaneity of the process.

Figure 2

The figure on the left schematically shows the enthalpy changes accompanying the three
processes that must occur in order for a solution to form: (1) separation of solute
molecules, (2) separation of solvent molecules, and (3) interaction of solute and solvent
molecules. The overall enthalpy change, DHsoln, is
the sum of the enthalpy changes for each step. In the example shown, DHsoln
is slightly positive, although it can be positive or negative in other cases.

The figure on the right schematically shows the large, positive entropy change, DSsoln, that occurs when a solution is formed. (Although DSsoln is generally positive, this value could be negative
in certain situations involving the dissolution of strong ions.)

In general, if the solute and solvent interactions are of similar strength (i.e.,
both polar or both nonpolar), then the energetics of steps 1 and 2 are similar to the
energetics of step 3. Therefore, the increase in entropy determines spontaneity in the
process. However, if the solute and solvent interactions are of differing strength (i.e.,
polar with nonpolar), then the energetics of steps 1 and 2 are much greater than the
energetics of step 3. Hence, the increase in entropy that can occur is not enough to
overcome the large increase in enthalpy; thus, the dissolution process is nonspontaneous.

To illustrate the importance of DH and DS
in determining the spontaneity of dissolution, let us consider three possible cases:

The dissolution of a polar solute in a polar solvent.

The polar solute molecules are held together by strong dipole-dipole
interactions and hydrogen bonds between the polar groups. Hence, the enthalpy change to
break these interactions (step 1) is large and positive (DH1>0).
The polar solvent molecules are also held together by strong
dipole-dipole interactions and hydrogen bonds, so the enthalpy change for step 2 is also
large and positive (DH2>0). The polar groups of
the solute molecules can interact favorably with the polar solvent molecules, resulting in
a large, negative enthalpy change for step 3 (DH3<0).
This negative enthalpy change is approximately as large as the sum of the positive
enthalpy changes for steps 1 and 2; therefore, the overall enthalpy change (DH1+DH2+DH3) is small. The small enthalpy change (DH),together with the positive entropy change for the process (DS), result in a negative free energy change (DG=DH-TDS) for the process; hence, the
dissolution occurs spontaneously.

The dissolution of a nonpolar solute in a polar solvent.

The nonpolar solute molecules are held together only by
weak van der Waals interactions. Hence, the enthalpy change to break these interactions
(step 1) is small. The polar solvent molecules are held together by
strong dipole-dipole interactions and hydrogen bonds as in example (a), so the enthalpy
change for step 2 is large and positive (DH2>0).
The nonpolar solute molecules do not form strong interactions with the polar solvent
molecules; therefore, the negative enthalpy change for step 3 is small and cannot
compensate for the large, positive enthalpy change of step 2. Hence, the overall enthalpy
change (DH1+DH2+DH3) is large and positive. The entropy change for the
process (DS) is not large enough to overcome the enthalpic
effect, and so the overall free energy change (DG=DH-TDS) is positive. Therefore, the
dissolution does not occur spontaneously.

The dissolution of a nonpolar solute in a nonpolar solvent.

The nonpolar solute molecules are held together only by
weak van der Waals interactions. Hence, the enthalpy change to break these interactions
(step 1) is small. The nonpolar solvent molecules are also held together only
by weak van der Waals interactions, so the enthalpy change for step 2 is also
small. Even though the solute and solvent particles will also not form strong interactions
with each other (only van der Waals interactions, so DH3
is also small), there is very little energy required for steps 1 and 2 that must be
overcome in step 3. Hence, the overall enthalpy change (DH1+DH2+DH3) is small.
The small enthalpy change (DH), together with the positive
entropy change for the process (DS), result in a negative free
energy change (DG=DH-TDS) for the process; hence, the dissolution occurs spontaneously.

The principles outlined in the green box above explain why the
interactions between molecules favor solutions of polar vitamins in water and nonpolar
vitamins in lipids. The polar vitamins, as well as the polar water molecules, have strong
intermolecular forces that must be overcome in order for a solution to be formed,
requiring energy. When these polar molecules interact with each other (i.e., when the
polar vitamins are dissolved in water), strong interactions are formed, releasing energy.
Hence, the overall enthalpy change (energetics) is small. The small enthalpy change,
coupled with a significant increase in randomness (entropy change) when the solution is
formed, allow this solution to form spontaneously. Nonpolar vitamins and nonpolar solvents
both have weak intermolecular interactions, so the overall enthalpy change (energetics) is
again small. Hence, in the case of nonpolar vitamins dissolving in nonpolar (lipid)
solvents, the small enthalpy change, coupled with a significant increase in randomness
(entropy change) when the solution is formed, allow this solution to form spontaneously as
well. For a nonpolar vitamin to dissolve in water, or for a polar vitamin to dissolve in
fat, the energy required to overcome the initial intermolecular forces (i.e., between the
polar vitamin molecules or between the water molecules) is large and is not offset by the
energy released when the molecules interact in solution (because there is no strong
interaction between polar and nonpolar molecules). Hence, in these cases, the enthalpy
change (energetics) is unfavorable to dissolution, and the magnitude of this unfavorable
enthalpy change is too large to be offset by the increase in randomness of the solution.
Therefore, these solutions will not form spontaneously. (There are exceptions to the
principle "like dissolves like," e.g., when the entropy decreases when a
solution is formed; however, these exceptions will not be discussed in this tutorial.)

In general, it is possible to predict whether a vitamin is fat-soluble or water-soluble
by examining its structure to determine whether polar groups or nonpolar groups
predominate. In the structure of calciferol (Vitamin D2), shown in Figure 3
below, we find an -OH group attached to a bulky arrangement of hydrocarbon rings and
chains. This one polar group is not enough to compensate for the much larger nonpolar
region. Therefore, calciferol is classified as a fat-soluble vitamin.

Figure 3

This is a 2D ChemDraw representation of the structure of calciferol, Vitamin D2.
Although the molecule has one polar hydroxyl group, it is considered a nonpolar
(fat-soluble) vitamin because of the predominance of the nonpolar hydrocarbon region.

Structures and Functions of Vitamins

Table 1, below, shows the structures and functions of several fat- and water-soluble
vitamins. To view a larger representation of the2D and 3D structures,
click on the name of the vitamin. To view and rotate the vitamin
molecules interactively using RASMOL,
please click on the three-dimensional structures for the coordinate
(.pdb) file.

Table 1

The 2D representations shown in this table were drawn using CS ChemDraw Pro, and the 3D
coordinates were obtained by MM2 minimization using CS Chem3D Pro.

Note: The 2D and 3D representations for each vitamin are drawn from
the same view. The 3D (but not the 2D) representations are all drawn to the same scale. In
the 3D representations, carbon atoms are gray, hydrogen atoms are light blue, oxygen atoms
are red, and nitrogen atoms are dark blue. The coordinates for the 3D representations were
obtained from molecular-modeling calculations, and the images were rendered using SwissPDB
Viewer and POV-Ray (see References).

Olestra and Vitamin Solubility

The solubility properties of vitamins determine how well they will be absorbed by the
body. Water-soluble vitamins can easily enter the bloodstream by diffusion, since the
stomach contents, extracellular fluid, and blood plasma are all aqueous solutions.
Fat-soluble vitamins must be consumed together with dietary fat to be absorbed. The
vitamins are first dissolved in the dietary fat. Then, bile released from the gall bladder
acts like a detergent and allows the fat (with the vitamins dissolved in it) to be
solubilized in micelles. (Recall the discussion of detergents and micelles from the
"Membranes, Proteins, and Dialysis" experiment.) However, some newly-developed
food products, unfortunately, have been found to disrupt this pathway for absorbing
fat-soluble vitamins in the body.

In recent years, many types of "fat-free" foods have come into the
marketplace. One such type of these foods contains artificial fats that are substituted
for the natural fats and oils found in the foods. These artificial fats add no fat or
calories to the diet, because they are not digested or absorbed by the body. The main
artificial fat commercially in use is Olestra. Olestra is marketed under
the name Olean by Proctor and Gamble, Inc. It is a synthetic sucrose ester that is not
digested or absorbed by the body. How does this work? Olestra, like natural fat, has
nonpolar hydrocarbon chains. But whereas fat has only three such chains attached to a
glycerol molecule (and thus is known as a "triglyceride"),
Olestra contains eight such chains attached to a sucrose molecule. (Refer to the figure on
membrane structure in the "Membranes and Proteins" experiment for the structure
of glycerol.) To digest natural fats in the body, lipase (an intestinal enzyme that breaks
down lipid molecules) removes the hydrocarbon chains from the glycerol. The hydrocarbon
chains are then emulsified (incorporated into micelles) with bile, and absorbed into the
bloodstream. Because Olestra has so many hydrocarbon chains, there is not enough room for
lipase to reach the place where they are attached to the sucrose, and so the side chains
cannot be removed. Therefore, the nonpolar Olestra molecule is too large to form
absorbable micelles, so it passes through the intestinal tract, undigested and unabsorbed
by the body. Olestra has been approved by the FDA for use in savory snacks, such as potato
chips.

Unfortunately, Olestra may not be as healthy as it first sounds. It has been shown to
cause gastrointestinal symptoms including abdominal discomfort, flatulence, and changes in
stool consistency. More importantly, it interferes with the absorption of fat-soluble
vitamins from food when present in the small intestine at the same time as other foods.
Because it is nonpolar, Olestra can dissolve fat-soluble vitamins. Hence, Olestra in the
small intestine competes with fat-containing micelles in the intestine for absorption of
fat-soluble vitamins. Anything the Olestra absorbs is carried out of the body with it and
is therefore not available for absorption by the body. Adding more fat-soluble vitamins to
food containing Olestra seems to be effective in preventing Olestra from depleting the
body's supply of fat-soluble vitamins. However, long-term studies are not yet conclusive
on the effects of continued ingestion of Olestra on humans.

Questions on Vitamin Solubility

For each vitamin in Table 1 in the tutorial, tell whether the vitamin is fat-soluble or
water-soluble. Briefly, explain your reasoning.

Vitamin C (ascorbic acid) is an anti-oxidant that helps keep cell membranes and other
lipids (fats) intact. When the lipids that constitute the membrane oxidize (lose
electrons), the membrane breaks down and no longer functions correctly. It is believed
that oxidation of these lipids is linked to aging, cancer, and cardiovascular disease. The
effectiveness of vitamin C in vivo (within a biological system) to prevent this oxidation
(i.e., to be an antioxidant) may be limited by its ability to permeate the biological
membranes.

Liu et. al. studied the effectiveness of vitamin C as an antioxidant. They
synthesized a derivative of vitamin C called ascorbyl-6-palmitate and discovered that this
derivative has increased effectiveness as an antioxidant in vitro (outside of biological
systems). Below is the structure of ascorbyl-6-palmitate (Figure 4). Briefly, explain in
terms of molecular structure and interactions why ascorbyl-6-palmitate should be more
effective.

Figure 4

This is the 2D ChemDraw structure of ascorbyl-6-palmitate, which was synthesized by Liu
et. al.

Mineral Solubility

Most minerals in the diet are in the form of water-soluble salts. When these salts
dissolve, they dissociate into aqueous cations and anions. It is customary to describe the
solubility of these salts (i.e., the solubility of minerals) quantitatively, as described
below.

Quantitative Measures of Mineral Solubility (Ksp and S)

To quantify the solubility of the ionic salts containing dietary minerals, two distinct
quantities are used: the solubility product, Ksp, and the solubility,
S. (These are the same quantities that you determined for calcium hydroxide in the
experiment.) The solubility product (Ksp) is the equilibrium constant of the
dissociation reaction of the mineral-containing salt in water. Hence, Ksp is a
constant at a given temperature. The solubility (S) of a mineral salt is the amount of the
salt that is dissolved per unit volume. This quantity may vary, depending on the
conditions. For instance, phytic acid from grain can bind to Zn2+ ions, making
these ions unavailable to the body. Suppose that you take a zinc supplement in the form of
ZnSO4. For your body to absorb the zinc, this compound must dissociate into Zn2+
and SO42-, as shown in Equation 1.

(1)

The equilibrium constant for this dissociation, Ksp, is a constant given by
the Law of Mass Action: Ksp = [Zn2+][SO42-].
If you also consume a large amount of phytic acid with the supplement, the phytic acid
will, in effect, remove free Zn2+ ions from solution. How does this affect the
solubility of ZnSO4? According to Le Châtelier's Principle (described in the "pH Regulation During Exercise"
tutorial), this will shift the equilibrium in Equation 1 toward the ions, and so the
solubility of ZnSO4 would increase (in order to keep the product of the Zn2+
concentration and the SO42- concentration constant).

Although the body's absorption of minerals depends in large part on their
solubility, we must be very careful not to equate solubility of the salt containing a
mineral with absorption of that mineral. In the example with zinc and phytic acid
described above, the absorption of zinc decreases with phytic acid even though the
solubility of zinc sulfate is increased. This is because the zinc is not present as the
free ion in solution; rather it is bound to phytic acid and is therefore unavailable for
absorption by the body. One way to overcome the problem of poor zinc absorption due to
phytic acid is to eat leavened, rather than unleavened bread. When yeast is used to make
bread rise, it destroys the phytic acid, and so the Zn2+ ions remain free in
solution to be absorbed by the body.

Calcium in the Body

Our bodies contain a staggering 1200 g of calcium. Only 1% of this calcium is in the
body fluids (the extracellular fluid, the blood, and the cellular fluid). The calcium in
the blood is important for a number of functions, including blood clotting, transmission
of nerve impulses, muscle contraction, stability of cell membranes, and cell metabolism.
The remaining 99% of the calcium in the body is contained in the bones in the compound hydroxyapatite,
Ca10(PO4)6(OH)2. This mineral provides the
structural integrity of the skeleton.

The calcium in the body fluids can exist in three forms: (1) as the
free cation Ca2+ (about 50% of the calcium in the fluids), (2) bound to
proteins (about 40% of the calcium in the fluids), and (3) complexed with other ions
(about 10% of the calcium in the fluids). Of these three, the free cation is the most
important for the physiological functions described in the paragraph above, and its
concentration must be carefully maintained. For instance, muscle contraction is initiated
by a sudden increase in calcium concentration in the muscle cells. Normally, this increase
in Ca2+ concentration is triggered by a nerve impulse; however, if the
"resting" Ca2+ concentration inside the muscle cells becomes too
large, the muscles will contract without the internal nerve signal to trigger an increase
in the concentration. The Ca2+ concentration in the extracellular fluid is kept
at approximately 10-3 M, and the Ca2+ concentration inside the cells
is kept at approximately 10-6 M. The body has several mechanisms to maintain
these ion concentrations. The cells have channels and pumps that regulate the flow of
calcium ions between the cells and the extracellular fluids via the cell membrane. In
addition, the calcium ions can be removed from or bound to the calcium-binding proteins in
order to increase or decrease, respectively, the free-ion concentration.

The two mechanisms for Ca2+-concentration maintenance described above
involve only exchange between the different forms of calcium storage in the fluids. What
happens if the overall amount of calcium in the fluids gets too low? In this case, calcium
can be supplied from two sources. (1) Calcium can be consumed in the diet, dissolved, and
absorbed into the blood. The normal calcium dietary requirement for an adult is
approximately 1 gram (1000 mg) per day. (Women and young people may need to consume even
more than 1 g of calcium per day.) (2) Calcium can be removed from the bones in order to
increase the Ca2+ concentration in the fluids. Hence, if too little calcium is
supplied in the diet, the body will take the calcium it needs from the bones. (Recall
equilibrium and Le Châtelier's Principle.) If this borrowing from the bones' calcium store continues over
time, bone mass will decrease, resulting in the condition known as osteoporosis.

Hence, it is clear that we must consume an adequate amount of calcium in the diet in
order to minimize loss of bone mass. But not all of the calcium that we consume ends up in
our body fluids. In fact, we only absorb 30% of the calcium that we consume, on average.
Several factors influence the absorption of the calcium that we consume. Two requirements
must be met in order for calcium to be absorbed: (1) it must be dissolved in the
intestine, and (2) it must pass through the intestinal walls into the body fluids. Some of
the most important factors are listed below:

Factors controlling the solubility of calcium in the intestine

The form of the dietary calcium affects calcium solubility. Different calcium salts have
different solubility products and therefore different solubilities in the intestinal
environment. Calcium citrate, for instance, is more soluble than calcium carbonate.

The pH of the intestinal tract affects calcium absorption. Most of the calcium absorbed
in the body is absorbed in the upper intestine, where the pH is low due to stomach acid
entering the intestine. Calcium requires a pH of less than 6 in order to enter solution as
Ca2+.

Factors controlling the absorption of dissolved calcium

Vitamin D stimulates intestinal calcium absorption.

Parathyroid hormone (PTH) also promotes calcium absorption. (PTH has two major
mechanisms for promoting the absorption of calcium: directly, by increasing Ca2+
reabsorption in the kidneys, and indirectly, by stimulating the activation of vitamin D.)
When the concentration of Ca2+ drops, the parathyroid gland releases PTH to
help bring the Ca2+ concentration back up.

Questions on Mineral Solubility

What effect does the addition of phytic acid have on the solubility product (Ksp)
of ZnSO4 in the body?

As explained in the list of "factors controlling the solubility of calcium in the
intestine" in the tutorial, the form of dietary calcium affects the solubility of the
calcium. The solubility product (Ksp) for calcium carbonate (CaCO3)
is 8.710-9. The solubility product (Ksp) for calcium fluoride
(CaF2) is 3.610-8. If 1.00 mol of each substance (CaCO3
and CaF2) were dissolved separately in 1.00 L of water, which solution would
have the greater concentration of calcium ions (i.e., which substance is more soluble)?
Show how you came to your conclusion.

Summary

The nutrients required by our bodies must be dissolved, and then absorbed by the body
if they are to be used. The solubility of nutrients is determined by the molecular
properties (e.g., polarity) of the nutrients. It is often useful to quantify the
solubility of nutrients, in terms of the amount of the nutrient that is dissolved per unit
volume. Although dissolution is a necessary step for nutrients to be absorbed, absorbance
depends on more than the solubility of the nutrients. Certain substances in the digestive
tract, such as Olestra and phytic acid, can interfere with the absorbance of some
nutrients even if the nutrients are dissolved; other substances, such as vitamin D, can
enhance nutrient absorption. All of these processes are governed by fundamental chemical
properties and principles, such as polarity, molecular structure, intermolecular
interactions, thermodynamics, and equilibrium.

Additional Links:

Vitamin Update provides information
about the functions of each vitamin in the body, as well as nutritional information on
vitamin consumption, and periodic news updates on vitamin research.

The Olean promotional homepage provides access to a
large body of research documenting the effects of Olestra consumption and how Olestra
works. This informative site also contains a search engine to locate information of
particular interest.

This site from Ball State University explains how the structure of Olestra
allows it to taste like fat but not be absorbed by the body. It also includes interactive molecular
representations that can be viewed using Chemscape Chime. To download CHIME, click here.

Virgin Earth (a supplement company) has an excellent site on minerals that contains a quiz to
determine if you might be deficient in one or more minerals, as well as information about
mineral interactions and mineral depletion from the soil.

Acknowledgements:

The authors thank Dewey Holten, Michelle Gilbertson, Jody Proctor and Carolyn
Herman for many helpful
suggestions in the writing of this tutorial.

The development of this tutorial was supported by a grant from the Howard Hughes
Medical Institute, through the Undergraduate Biological Sciences Education program, Grant
HHMI# 71199-502008 to Washington University.